Calculating approx. Values in km3, m3, cubic miles, and gallons

1. The problem statement, all variables and given/known data
Earth’s radius is 6370 km and approximately 70% of
its surface is covered with water, to an average depth
of 12,000 feet. Calculate the approximate volume, in
km
3
, m
3
, cubic miles and gallons of Earth’s oceans.

2. Relevant equations

3. The attempt at a solution

I was told to find the area and to multiply by the avrg depth but i am still confused.

Welcome to PF.
You can calculate the total volume of the earth and then subtract the non water portions.
1. Calculate total volume in desired unit.
2. Calculate volume of "inner" portion.
3. Subtract inner portion from total volume then multiply by 0.7 to account for 70% water coverage.